This invention relates to an optical transmission system, and more particularly to such a system in which light is transmitted over a single optical fibre at a number, possibly a large number, of different wavelengths, with each wavelength, or ‘colour’ of light carrying a separate optical communication channel. Such a system is often referred to as a Dense Wavelength Division Multiplex (DWDM) system. One of the effects of fibre transmission characteristics, such as non-linearity, is to degrade the shape of pulses of light transmitted within each channel and to generate undesirable side bands at wavelengths which could differ from the wavelength which is launched at the beginning of a fibre. These side bands can coincide with adjacent transmitted wavelengths (ie slightly different colours) and so interfere with adjacent channels to cause corruption of the pulses transmitted at these other wavelengths.
It is necessary to minimise the channel spacing in DWDM systems so as to accommodate a large number of channels in the available overall bandwidth. In order to maximise the use of the available bandwidth, equal channel spacing on a grid specified by the ITU is frequently adopted, but equal channel spacing is sensitive to unwanted side bands as these can appear as in-band crosstalk disturbances that reduce the signal to crosstalk ratio.
The suppression of all in-band crosstalk terns implies an unequal channel spacing allocation that requires a prohibitively large system bandwidth.
The present invention seeks to provide an improved optical transmission system.